We have produced a map of velocity covering much of the Siple Coast ice streams. The map confirms earlier estimates of deceleration on Whillans Ice Stream. Comparison with bed elevation data indicates that subglacial topography and the location of consolidated sediment play a strong role in determining the location of the tributaries feeding the ice streams. Force balance estimates based on these data indicate that the tributaries have beds nearly an order of magnitude stronger than those beneath many of the ice streams. We have used a theoretical analysis to examine the controls on fast flow. This analysis suggests that ice plains (very wide ice streams) are inherently unstable. This instability may be responsible for the current deceleration on the Ice Plain of Whillans Ice Stream and the shutdown of Ice Stream C 150 years ago. Thinning-induced reductions in driving stress may also explain some of the observed deceleration, particularly in upstream areas. The active portions of Ice Stream C coincide well with the areas where we estimate that melt should be taking place. Current topography and inferences of large thickening following a shutdown suggest the upstream migration of a stagnation front that initiated at the ice plain. Uncertainty remains about the basal conditions on Ice Stream D, while the basal resistance on Ice Stream E is large enough to ensure basal melting.